The invention relates to selective catalytic reduction systems for internal combustion engine exhaust, including for use in industrial processes and in mobile and stationary diesel, natural gas and other engine applications.
Selective catalytic reduction (SCR) of nitrogen oxides (NOx) with urea or ammonia solution is known in industrial processes as well as in stationary diesel engine applications. In the SCR system, urea or ammonia solution is injected into the exhaust gas flow stream before the catalyst. Water in the droplets from the injection solution evaporates, leaving urea behind, which decomposes and hydrolyzes into ammonia, i.e. ammonia is produced through the process of droplet evaporation and urea chemical decomposition. NOx reacts with the ammonia and is catalytically reduced in the presence of such ammonia.
Thus far, SCR catalysts have not been successfully commercialized for mobile applications because of complexity. One of the many obstacles is that the limited space available in mobile applications makes it hard to evaporate droplets and decompose and hydrolyze the urea completely and achieve a high enough ammonia to NOx ratio. At low diesel engine exhaust temperatures and high gas flow velocities (e.g. about 20 meters per second), a distance of several meters (i.e. a time of 0.1 to 0.3 seconds) between the injector and the catalyst is required for the aqueous urea solution spray to vaporize, for urea to decompose and hydrolyze into ammonia and carbon dioxide, and for the ammonia to become uniformly distributed across the flow front before entering the catalyst. In order to be practical, this distance must be shortened to less than about 0.5 meters. Uniformity of the ammonia distribution is one of the key factors affecting the performance of the SCR system.
The present invention provides a selective catalytic reduction system which may be used in mobile applications. Evaporation rate is accelerated as well as the ammonia diffusion rate, in combination with enhancement of the noted ammonia mixing for reducing NOx.